Silicon carbide has been an attractive potential material for use in electronic devices for many years. Because silicon carbide has no low temperature phase changes, a comparatively large energy bandgap, a high breakdown field strength, a high saturated electron drift velocity, and chemical resistance to most species commonly found in the environment of electronic device processing, silicon carbide has been an enticing, though largely unrealized candidate material for applications involving rigorous mechanical and thermal shock, as well as high operating temperatures, high power, high frequency, and intense radiation.
Many types of silicon carbide made by a variety of processes have been tried as candidate substrate materials. Previous efforts include the use of substrates made from the consolidation of finely divided ceramic powders (sintering), substrates made by various reactions of silicon metal, carbon, and other additives (sometimes referred to as the "Carborundum Process"), and substrates made by seeded sublimation (single crystal).
A silicon carbide ceramic monolith made by consolidation of finely divided particles, for example by "sintering," is too porous to support circuit lines measuring less than 0.25 microns wide. Though significant portions of the monolith may exhibit pores no larger than 1 or 2 microns, occasional larger pores measuring 75 microns across will occur which cannot be reduced in size or number by controlling the manufacturing process and therefore must be detected at great cost by measuring each part after the cost of manufacturing has been incurred.
Silicon carbide made by consolidating or reacting finely divided particles produces the wrong crystallographic form to support an electronic device. Usually hexagonal or rhombohedral or a combination thereof in crystal shape, commonly referred to as "alpha SiC", these crystals do not match the coefficient of thermal expansion of epilayers closely enough to avoid undesirable stacking faults bounded by partial dislocations. Device quality is limited. Researchers have shown, however, that by coating an alpha SiC wafer or substrate with a cubic "beta SiC" epilayer or by doping the alpha SiC substrate with aluminum, boron, nitrogen, or phosphorus species, the "crystal shape memory" imparted by the alpha SiC to a subsequent beta SiC epilayer can be blurred so device quality beta SiC is formed.
Large single crystal silicon carbide is made by a very slow, capital intensive process entailing the sublimation of silicon carbide onto a small single crystal seed. The process for large single crystal to date has been unable to produce the standard diameter wafers currently preferred by device manufacturers to minimize fabrication costs. U.S. Pat. No. 4,866,005, incorporated herein in its entirety by reference, describes one such method of growing large single crystals. The largest single crystal silicon carbide substrates commercially available measure only 50 mm diameter compared to the 200 mm or 300 mm diameter substrates normally used in industry. A 50 mm substrate has only 1,962.5 mm.sup.2 of potentially useable surface area on one side compared to 31,400 mm.sup.2 of potential surface area for a 200 mm substrate. Since it costs about the same for a device manufacturer to process a 200 mm wafer as to process a 50 mm wafer, devices made using a 200 mm wafer cost only 6.25 percent as much as devices made using 50 mm wafers. Because 50 mm substrates are only available in small quantities costing more than $2,000 per wafer, using a 200 mm substrate would represent significant savings to a device manufacturer even if the 200 mm substrate sold for $2,000.
Other issues arise from the use of large single crystal silicon carbide. Many of these issues are described by U.S. Pat. Nos. 4,912,063 and 5,200,022, incorporated entirely herein by reference. Single crystal silicon carbide is made at temperatures favoring the formation of the .alpha.SiC polytype. The "alpha" silicon carbide (.alpha.SiC) polytype is a collective reference to some 170 types of hexagonal and rhombohedral crystal shape classifications. The most common is the 6H hexagonal polytype. Most .alpha.SiC polytypes are separated by small thermodynamic differences that are difficult to control in a temperature dependent manufacturing process like that used to make large single crystals.
Beta SiC (.beta.SiC) is the only cubic crystalline polytype. Theoretically an excellent semiconductor material, deposition of .beta.SiC with oriented crystals (also referred to as epitaxy, epilayers, or "thin films") on various ceramic or silicon metal substrates has been attempted on various occasions over many years. The difficulty of producing consistently high quality, low carrier concentration .beta.SiC epitaxial layers has prevented commercial exploitation.
In order to provide a hospitable surface on which to support an epitaxial .beta.SiC layer, substrates are usually made from large .alpha.SiC single crystals by sawing the crystal into wafers along a [0001] surface plane, then lapping and polishing the wafers to a smooth surface. The {0001} crystallographic surface plane provides a surface that matches the Coefficient of Thermal Expansion (CTE) of the (111) surface of .beta.SiC. Low defect .beta.SiC epilayers have thus been applied to single crystal .alpha.SiC. The process of lapping and polishing single crystal .alpha.SiC substrates, however, typically cuts through a multitude of surfaces to expose surface features associated with higher surface energy than a broad, flat single surface. A surface intersecting multiple crystalline lattice is planes creates a series of physical "steps." Multiple steps transmit three dimensional lattice information to an epitaxial layer usually causing formation of a mixture of alpha and beta polyforms in the epitaxial layer, thereby making the subject layer unsuitable for high quality devices.
Silicon carbide substrates can be used to support magnetic media in hard disk drives (U.S. Pat. No. 5,623,386). The mechanical requirements for disk drive substrates rotating at up to 10,000 rotations per minute with heads gliding less than 0.025 microns above substrate surfaces surpass the mechanical requirements of chip substrates. Magnetic media needs a substrate without surface defects such as pits that cause "dead spots" in the media or surface features exposing different lattice planes that cause variations in the magnetic media, commonly referred to as "noise." The size of surface pits or defects on disk drive substrates which can be tolerated, however, is considerably larger than the size of surface defects which can be tolerated for high quality electronic devices like chips (approximately 2 microns for disk drive substrates versus 0.1 microns for a high quality integrated chip for example). Apart from the issue of exposing high energy lattice positions that might cause undesirable crystallographic forms, media used for disk drives is much thicker than thin film coatings used for chips and can more easily "bridge" surface defects. Magnetic media is also different from the thin films of electrical devices in the way it functions. Magnetic media does not require electrical continuity to function. Data bits in magnetic media are magnetic domains, the polarity of which determines whether an individual bit is either a 0 or a 1 binary code.
It has nevertheless been generally accepted that polycrystalline SiC monoliths will not support epitaxial SiC thin films of the purely beta polyform required for some high quality electrical devices.